1. Field of the Invention
The present invention relates to connectors for joining oilfield tubulars. More specifically, the present invention relates to an improved coupling and threading arrangement and design for securing tubular connectors together with a connection that resists cyclic bending loads.
2. Description of the Prior Art
Tubular strings of pipe used in the drilling and completion of oil wells are subjected to both axial and lateral stresses during their installation and operation. Lateral forces, in the form of side bending loads, are encountered, for example, in riser pipe strings that extend from the water bottom to a drilling or production structure at the water surface. Tubulars employed in these situations are typically provided with integral, shouldering-type connections, and/or weld-on connections, that are particularly well suited for resisting the fatigue-inducing effects of the cyclical side loading. Connections of this type, which have been developed to provide high fatigue resistance, include tool joints and specialty connectors for risers. These connections, as compared with conventional threaded and coupled connections, are substantially more expensive to fabricate and typically require a larger lateral dimension to accommodate the connection design.
Conventional threaded and coupled pipe is subject to rapid fatigue failure when exposed to the cyclical side loading conditions that are designed to be met by the more expensive integral joint and welded joint designs. In a typical cyclical side loading failure, the pin of a conventional threaded and coupled connection experiences a fatigue-induced failure at a point adjacent the coupling end. Because of the conventional thread and coupling construction, side loading cyclical forces are directed into the pin at the end of the coupling and cooperate with the thread structure on the pin itself to produce fatigue failures. The concentration of stresses results in part from the structural resistance encountered at the end of the coupling with the underlying pipe pin material. The threads in the same area act to concentrate the bending stresses in the pin thread root where the failure ultimately occurs.
Another feature of some conventional, interference fit coupled and threaded connections, such as API buttress-style thread forms, is that the threads engage along only one thread flank upon makeup. This type of connection must completely unload the contacting flank, undergo relative movement between the pin and the coupling until the opposite flanks contact, and then transfer load to the newly contacting flank. Repeated, cyclical side loading and load transfers make these connection types especially susceptible to fatigue failures.
Threaded and coupled pipe is provided with a design that allows it to be used in cyclical, side loaded applications with increased resistance to fatigue damage to replace more costly upset and welded connections. In one embodiment, incorporating multiple features of the invention, the pin (or male) thread of the connection is machined along a tapered path such that the diameter of the thread is increasing at a constant rate away from the end of the pipe. The threads are machined at a constant taper angle until they completely vanish from the outside diameter (OD) surface of the pipe. The threads of the pin and coupling have stab and load flanks that engage with each other upon makeup of the coupling to the pin. The coupling threaded area is longer than a standard coupling so that the coupling completely engages all of the pin threads and extends beyond the thread run-out on the pipe OD. The coupling thread length ensures that the threads on all pins having thread lengths within accepted ranges will be fully engaged by coupling threads. The coupling is further modified by providing a long external bevel on the OD starting at the face of the coupling and extending to a location at or beyond the last point of full depth pin threads. In the engaged position, the end of the coupling extends beyond the vanish point of the pin threads and tapers to a relatively thin face width. The width of the face is made as thin as practical without rendering it overly prone to mechanical handling damage. While the bevel, as described, is conical, it will be understood that other shapes, e.g., an annular, smooth shallow concave depression in the conical surface, could be employed. In general, there must be a transitionxe2x80x94i.e., thinner to thickerxe2x80x94so long as the transition is without any abrupt changes in geometry.
The length and starting point of the taper are determined as a function of the pipe OD and the pipe wall thickness. The larger the pipe OD, and the thicker the pipe wall, the longer the external taper on the coupling. The length (L) of the bevel can be considered in the context of a stiffness gradient spanning the distance between the end or face of the coupling and the point on the OD of the coupling where the bevel ends, which will be recognized as being a point intermediate the ends or faces of the coupling. Stiffness of a tubular body is defined as [OD4 (in.)xe2x88x92ID4 (in.)]xc3x97xcfx80/64 where ID is the internal diameter of the tubular body and OD is the outside diameter of the body. It is desired that the stiffness gradient of the bevel be from 0.15 to 0.4 per inch of length of the bevel. To determine the gradient, assume the stiffness of the coupling at the end or face is S1, the stiffness of the pin or pipe at that point is S2, and the ratio of S1 :S2 equals R1, whereas the stiffness of the coupling where the bevel intersects the coupling OD is S3, the stiffness of the pin at that point is S4, and the ratio of S3 :S4 is R2. The gradient is thus (R1-R2)/L and is from about 0.14 to about 0.4 per inch of length of the bevel. With respect to the radial thickness of the face or end of the coupling, it is desired that the stiffness ratio at that point, defined above as ratio of S1 :S2, be 0.1 to 0.4.
From the foregoing it may be appreciated that an important object of the invention is to provide a threaded and coupled connection for tubular members forming a pipe string that resists fatigue damage in cyclical side loading applications.
Another object of the present invention is to provide a threaded and coupled connection having the resistance to side loading and fatigue damage normally limited to connections such as tool joints and specialty connections employing upset pipe and connections welded onto the upset pipe end.
Yet another object of the present invention is to provide a low-cost coupling for securing the threaded ends of pipe together in a connection that resists cyclical side loading fatigue damage.
It is also an object of the present invention to provide a threaded and coupled connection for a pipe subjected to cyclical side loading that can be machined on conventional equipment used for coupled and threaded pipe, has relatively small lateral dimensions, and can resist the effects of side loading without use of shouldering connections, or upset and welded connections commonly required for such applications.
An important object of the present invention is to provide a coupling having an internally threaded area that will mate with all of the pin threads of any pin having a thread form within a normally accepted range.
A related object of the present invention is to provide a pin thread on a pipe that extends to a vanish point on the pipe surface such that all of the pin threads will be engaged by the threads of a similarly threaded coupling having coupling threads that extend beyond the vanish point of the pin thread when the pin and coupling are fully engaged.
Another object of the present invention is to provide a thread profile for a pin and coupling connection in which the stab and load flanks of the threads engage simultaneously upon makeup to reduce stress concentration in a cyclically side loaded tubular pipe string.
Yet another object of the present invention is to provide a coupling that is longer than the standard coupling length and that includes an internal thread area that engages all of the pin threads and extends beyond the threaded area of the pin and over the unthreaded part of the pin to improve resistance to cyclical side loading of the threaded connection.